1. Field of the Invention
The invention disclosed herein relates to oil exploration and, in particular, to a method for using nuclear data to provide related indications of resistivity values.
2. Description of the Related Art
Water saturation and hydrocarbon saturation, its complement, are two of the most important petrophysical parameters. Connate water resistivity, has been one of the main inputs to all water saturation equations. Until now, water saturation has been measured almost exclusively from electric logs sensitive to bulk resistivity using one of a multitude of empirical functions. None of these functions has satisfactorily accounted for the many variables in downhole resistivity, resulting in great disarray in the industry and uncertainty in the final result.
Chlorine is an important part of the formations measured by wireline logging. It is present in at least three forms: as a solid part of a surrounding rock matrix, as an ion in the pore fluid, and as an ion in the formation fluid.
Salt content in the rock matrix is often known from other information such as core descriptions, mud logs, X-Ray fluorescence, X-Ray diffraction, and other wireline devices. The salt content is converted to a concentration of chlorine in kppm. In many wells, the salt content is effectively 0 kppm.
The chlorine concentration of the borehole fluid is often known directly from the mud report given by the drilling fluid operator. However, chlorine content is not always uniform in a wellbore due to salinity gradients and other driving forces. One method of knowing the chlorine content in a wellbore is the use of a wireline temperature and mud resistivity log.
Temperature and pressure have an effect on resistivity, temperature more so than pressure. In the preferred embodiment of this invention, the effects of pressure are considered negligible. The mud resistivity measurements can be converted to yield a constant temperature.
Some instruments that may be used downhole do not operate principally as a resistivity instrument. For example, a pulsed neutron instrument may be used to collect valuable data. Unfortunately, when the pulsed neutron instrument is in use, this may preempt resistivity measurements with a resistivity instrument.
Therefore, what are needed are techniques for generating forms of resistivity data from a pulsed neutron instrument. Preferably, the techniques are useful for identifying elemental constituents, such as chlorine.